Intrinsically Conductive Polymers (ICP) have been studied since at the latest the 1970's. The term "ICP" refers to organic polymers which have (poly)-conjugated .pi.-electron systems (e.g. double bonds, aromatic or heteroaromatic rings or triple bonds). Examples of such polymers are polydiacetylene, polyacetylene (PAc), polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh), polyisothionaphthene (PITN), polyheteroarylenvinylene (PArV), in which the heteroarylene group can be the thiophene, furan or pyrrole, poly-p-phenylene (PpP), polyphthalocyanine (PPhc) and the like, and their derivatives (formed for example from monomers substituted with side chains or groups), their copolymers and their physical mixtures. They can exist in various states, each described by different empirical formulae, which can generally be converted essentially reversibly into one or the other by (electro-) chemical reactions such as oxidation, reduction, acid/alkali reaction or complexing. These reactions are also occasionally known as "doping" or "compensation" in the literature, or can be regarded as "charging" and "discharging" in analogy with the electrochemical processes in batteries. At least one of the possible states is a very good conductor of electricity, e.g. has a conductivity of more than 1 S/cm (in pure form), so one can speak of intrinsically conductive polymers. These forms of the ICP are generally recognized as being polyradical cationic or anionic salts.
A good overall review of the intrinsically conductive polymers synthesized to date with a chemical structure suitable for the present objective, is to be found in Synthetic Metals, Issues 17, 18, and 19 (1986), and in Synthetic Metals, Issues 27, 28 and 29 (1988), which are hereby incorporated by reference.
Despite several potentially useful properties exhibited by ICPs, their use as conductive materials has been limited because they do possess some undesirable properties such as, poor processibility (no melting or glass transition temperature), poor solubility in the doped form and environmental instability. To be commercially useful, it is necessary to render these polymers processible by conventional techniques.
Several articles have appeared in the literature describing ways to overcome the processibility problem. These include attempts to synthesize soluble conductive polymers or precursors and/or polymerization of conductive polymers in situ thereby forming conductive polymer composites. Much of the known work on conductive composites using intrinsically conductive polymers such as polyaniline and polypyrrole has generally included electrochemical or chemical coating of the conductive polymer onto a polymeric substrate or electrochemical polymerization onto a swollen polymer using an appropriate solvent.
For example, U.S. Pat. No. 4,617,228 describes a process for making an electrically conductive composite by treating a porous substance such as fiberglass, with the monomer solution, and later treating with an oxidizing agent to produce a conductive composite containing an intrinsically conductive polymer. Similar techniques using a non-porous substrate and/or via solution of the monomer have been illustrated in U.S. Pat. No. 4,604,427 and Japanese Pat. No. JP 61,127,737. These composites have failed to yield highly conductive processible blends, and their preparation has proven to be cumbersome.
Blends of intrinsically conductive polymers with conventional, processible thermoplastics have been suggested to overcome processibility, such as shown in U.S. Pat. No. 4,935,164 (polymer blends), U.S. Pat. No. 4,929,388 (conductive pathways), International Patent Application WO 89/02155 and British Patent No. 2,214,511. These describe post polymerization processes in which the intrinsically conductive polymers are present in a dispersed phase in a thermoplastic matrix, yielding good processibility and high conductivity above a certain critical volume concentration of the dispersed conductive polymer phase. These processes present a generally useful procedure to process conductive polymers into various forms.
As used in the art and as employed herein, the term "polymer blend" is generally understood to mean macroscopically homogeneous mixtures of partly compatible or incompatible organic polymers. They always consist of two or more phases.
Nevertheless there is still a disadvantage resulting from the chemical nature of most of the conductive polymers which include protonic acids as "dopants". They react directly or indirectly with various functional groups present in the polymer matrices which are susceptible to reactions with acids or bases. More specifically, for instance, the basic nature of various homopolymers and copolymers of polyamides (nylons) provides a basic environment which leads to partial dedoping (compensation) of the conductive polymer and/or degradation of the polymer matrix. The same result appears to occur with several aromatic or aliphatic esters present in the various polyester homopolymers or copolymers (regardless of whether they are aromatic or aliphatic) which are used according to the above-mentioned procedures. Polymer matrices such as PVC homopolymers or copolymers, chlorinated polyethylene and similar polymeric matrices are used routinely with plasticizers which have been observed to react with conductive polymers thereby leading to compensation reactions.
With respect to thermoplastic vinyl compounds in general, it is known in the art to employ a plasticizer as a possible major component of a vinyl compound. The selection of the specific plasticizer is based on compatibility with the host polymer, cost and performance. In order for the plasticizer to be compatible, the solubility parameters of the polymer matrix and the plasticizer should match. Typically used plasticizers include phthalate esters such as dioctyl phthalate and dibutyl phthalate, polyesters, azelates, adipates, sebacates, organic phosphates and the like.
The "solubility parameter" is the measure of polarity of a material, and is defined as the square root of cohesive energy density (CED). Most commonly used plasticizers, such as esters have a solubility parameter in the range of 8 to 10 (cal/cm.sup.3).sup.1/2.
Ester-type plasticizers are those plasticizers which are formed by the reaction of an acid and an alcohol or compounds that contain the following structural unit: ##STR1## Typical examples of ester plasticizers include dioctyl phthalate, dibutyl phthalate and the like. Highly polar plasticizers are those with solubility parameters greater than 11(cal/cm.sup.3).sup.1/2. Typical of this class are sulfonamides, which are generally free of ester linkages.
Furthermore, polymers having acetate, propionate, or other ester groups, are present in various polymers of acrylates which are used in pure form or in polymer blends, as impact modifiers in polyvinyl chloride, polystyrenes and other polymers, have also been observed to react with intrinsically conducting polymers.
Therefore, a need exists for improving the presently available processing techniques of intrinsically conductive polymer blends.